Used Bike Guide – Engine/Fluids/Carb

Did the seller warm up the bike before you got there? (See if the engine cases are warm, but they might be hot, so be careful and don’t get burned. Engines will stay warm for a couple of hours; exhaust pipes get MUCH hotter much faster but cool quickly.) A pre-warmed engine might have been started & warmed-up to mask cold-starting problems, so this might be a good thing to check first… then you can let the engine cool down as you test other things, and get back to checking the engine after it’s had a little more time to cool. In particular, if the bike you’re going to look as is a kick-start, make sure you can kick-start the engine when it’s cold.

You’ll probably be able to sense heat radiating from a surface before you actually have to touch it, but when touching potentially hot surfaces, use the back of your hand. Your body’s reflex reaction to dangerous heat is more likely to pull your hand away if you use the back of your hand. (But don’t get into this situation in the first place! Be careful around hot surfaces, or surfaces that might possibly be hot. Use common sense.)

The engine should start uneventfully (with some choke*, if it’s cold) and sound reasonably good. If you hear obviously bad sound like loud clacking sounds or sounds like shaking a coffee can full of marbles, run away and don’t look back. The engine should rev smoothly off idle. Don’t redline the thing, but after it’s fully warmed up, twist the throttle and see what happens. Hesitation & stumbling = carburation problems.* A test ride will help you gauge whether or not these will be easy to live with. The throttle grip, when released, should snap closed sharply, no matter how the handlebars are turned. Try turning the bars full-lock left and right, and test cable action at both extremes as well as in the middle. Resistance at the extremes but not in the middle is probably just a cable routing issue. Half an hour of labor – if that – to fix. If the cable moves with resistance everywhere, the problem is probably the carbs, not the cables themselves. See below. While the bike is running, and in neutral, turn the bars — does the engine rev without even twisting the throttle? Cable routing problem. When you give the throttle a little blip with the bars turned all the way, does the engine rev and keep revving? Cable routing problem.

*=These comments refer to carbureted bikes. Some more modern bikes are fuel-injected: instead of carburettors, the bike is equipped with throttle bodies and fuel injectors. Fuel-injected bikes sometimes have a “fast idle” lever instead of a choke lever, but some detect the need for an enriched (choked) mixture by computer, and automatically adjust the fuel-injection accordingly. You should not experience any “carburation” problems with a fuel-injected bike, and if you do, they may be harder to correct than on a bike equipped with carburators.

Some bikes use a fuel pump which may need to build pressure before the bike will start. If you flip the ignition switch to “on” and hear a whirring sound from the gas tank, wait for it to finish before thumbing the start button. (If you don’t, and you know the bike has a fuel pump, they’re about £100 + 1-2 hours of labor to replace.)

If the bike has a centerstand, put the rear wheel in the air and try shifting throught the gears to make sure they all engage properly. Don’t spin the elevated rear wheel too fast — if the bike slips off the cenerstand, it’ll launch you into next week. Letting the bike idle and clicking through the gears is fine. Always keep the front brake applied when doing this, just in case.

The oil level should be visible through a sight glass or dip-stick, typically on the right side of the engine. Make sure the level is between the upper and lower edges of the glass (or marks on the stick) when the engine has been off for at least a few minutes and the bike is on level ground. Way too low or too high is very bad, but just outside the range probably hasn’t caused any damage. The surface level doesn’t have to be right in the middle, but it should be visible through the glass. See below for color analysis. Ask the owner when the oil was last changed. The owner better know. As far as frequency goes, at least every 5k miles or 6 months is fine, and always before storing the bike for a while (e.g., before the winter). this interval only applies for street bikes — dirt bikes should get oil changes much more frequently.)

Checking oil color… look through the sight glass. If your bike doesn’t have one, you’ll need to dip something down into the oil fill-up spot. Either use a dowel or popsicle stick, or roll up a paper towel. Pull it out and look at what color you’ve got:

If the throttle cable twists with a lot of resistance (and then won’t snap closed), there are a couple of possibilities, none of which is really good news:

The carbs may be hopelessly gunked up with gas and varnish. If the bike won’t start, that definitely points to this possibility (rather than either of the next two.) A good carb cleaning will either cost around £200 of shop labor or £5 + 1-3 hours of your time, depending on whether you have a shop do the work or you do the work yourself. (Warning: not for the inexperienced or mechanically faint of heart — there are lots of small and easily-confused parts — but if you’ve done it before, it’s not too bad.)

The handlebar itself may be slightly bent, preventing the twistgrip’s throttle tube from sliding well. Look very closely — sometimes it’s hard to tell unless you really scrutinize it (or remove the throttle tube.) Bent handlebars can cost £75 or more to fix, and are a good indication that the bike was crashed and may have other crash damage. Be on the lookout.

The throttle cables may partially seized, or simply routed improperly. This may mean that the carbs are fine. It’s very hard to check while you’re visiting a prospective acquisition, but try straightening cables or untwisting them and see if the behavior changes substantially. If straightening them or untwisting them makes them slide a little easier, they’re probably routed around the frame the wrong way (hamfisted home mechanic alert!), and they can be fixed fairly easily. If not, new cables will probably run you about £20 each, plus about half an hour of labor to install.

Some engines use air and oil for cooling, some are water cooled. The comments below about checking the coolant or worrying about coolant in the oil apply only to liquid-cooled models, not to air- or air/oil-cooled models.

If the oil has a white streaks in it (look at the sight glass) that’s water — beware! Water in the oil could be two things — condensation from the air in the engine, or a leak in the coolant system that’s letting water escape into the lubrication system. (Guess which one isn’t so bad and which one is really bad.) Condensation will burn off… let the bike run for a while (20-30 minutes?) and see if the white streaks in the oil are gone. If not, you’re probably looking at major engine work to replace gaskets (or worse.) Side note: two-stroke with milky white oil can be repaired much easier than four-stroke engines. (“Two stroke” is an engine configuration, and has nothing to do with how many cylinders the machine has. Two stroke bikes sound just like chainsaws, because chainsaws use two-stroke engines.)

Check coolant level. Find the radiator overflow bottle, and see if the coolant is between the “high” and “low” lines on the bottle. If you can’t find the coolant overflow bottle, trace the thin coolant tube back from the radiator cap assembly — it almost always goes to the coolant overflow bottle. If the coolant is clear (i.e., it’s water) or is a light pink, it may be an indication that the bike has been raced. (Roadracing organizations don’t allow the use of antifreeze, so race bikes run with plain water or plain water with a product called WaterWetter that makes the water pink.) This does not apply to dirt racebikes, which will probably have green coolant.

The coolant itself should be a neon green, not brown or even a murky green-brown. You’ll need to remove the radiator cap to check the coolant color, something you never want to do when the engine is still hot. If the radiator cap is hot (be careful!), do not open it — come back to this step later, when the engine’s had time to cool down. If you can safely open it:

Pure, clear water is bad — it’s at least an indication that the coolant system has been run without corrosion inhibitors, and also an indication that the bike may have been raced.

Pinkish-tinted water is also a possible indication that the bike has been raced.

Bright green coolant is good.

Brown-colored coolant either has rust in it (bad!) or oil in it (bad!). The former indicates that the insides of the engine have started rusting — run away! Oil in the coolant probably means trouble with the head gasket or the O-rings on the oil cooler (if the bike has one.) Bad head gaskets is Very Bad, failed O-rings is only a little Bad. I’d have a professional mechanic look at the bike so you know which it is. And/or consider giving up and looking at other bikes.

Finally, no coolant in the radiator is extremely bad — run away!

One other head gasket check… You won’t notice this unless you spend a fair amount of time with the bike, but a partially blown head gasket will allow the bike to consume coolant over time, which will gradually lower the coolant level in the overflow bottle. It’s OK for the bike to emit white smoke out the exhaust pipes as it’s warming up, but after it’s been running for a while and it’s nice & hot, the exhaust gasses should be invisible. White smoke coming from a hot bike is a sign that the head gasket is leaking badly.

Bikes should not emit blue smoke. White smoke (as mentioned above) is water burning off, blue smoke is oil burning. Why’s the oil burning? Either because the bike is a Harley or because its rings and/or valve stem steals are worn out. If the bike emits blue smoke, have a mechanic do a compression test or a leakdown test (see below.) Or give up and look for other bikes.

Side note: it is very normal for two-strokes to burn oil and thus emit blue smoke, since they’re designed to be lubricated by oil mixed into the gasoline. This smoke tends to go away as the two-stroke engine heats up, but they’re often called two-smokes for a reason. As noted above, two-strokes will sound like chainsaws.

Needless to say, I should think, fluids leaking from the engine are a Bad Thing. Probably just new gaskets, but possibly worse. If you don’t feel qualified to decide, I’d recommend having a mechanic give you his/her opinion, or simply giving up on the leaker.

Engine compression: engines are basically air pumps, and must seal tightly to work well. Engines that don’t seal well will be hard to start, will burn oil (blue smoke), and will have reduced power and fuel economy. Old engines will tend to exhibit this more than low-mileage ones, but young engines that have been abused may also have low compression numbers. Unless you know what you’re doing, have a shop do a compression test on the bike. It’s not a critical test, but it might give you some evidence one way or the other if you suspect that the bike may have been abused.

Dirt bikes and some older street bikes have kick-starters that enable you to spin the engine directly. So even if you don’t have a compression tester, you can at least test to see if you can feel some compression. If you spin the engine with the kick-starter and feel it get substantially harder to spin at certain points (almost like there there’s a “tight spot”) — that’s good: what you’re feeling is compression. If you spin the engine with the kick-start lever and it doesn’t really feel like there’s a tight spot, the engine is probably suffering from a serious lack of compression. Run away, or, if you have your heart set on it, have the bike checked out by a shop!